"We still have some work to do," said Nobuyuki Sakakibara, the RIZIN chairman.

The 20-year-old Nasukawa, speaking at a news conference alongside Mayweather, called it "probably the biggest event of my life so far."

He also vowed to end Mayweather's long string of victories.

Floyd Mayweather, right, of the U.S. claps as Japanese kickboxer Tenshin Nasukawa strikes a pose during a press conference in Tokyo. (Katsuya Miyagawa/Kyodo News via AP)

Nobody has defeated my opponent in the past, and I would like to be the man who makes history," Nasukawa said, speaking through an interpreter. "My punch can change history. And I will show that to you. So please stay tuned.